Power plants often require a pipe installation characterized by a metal pipe and means for supporting the pipe at axially interspaced positions. When the pipe is subject to thermal expansion and contraction movements in its axial direction, it is desirable that it be supported at positions as far apart as is possible without the pipe, because of its weight and dimensions and the elasticity of metal, sagging to an impractical extent between its supported positions.
An example of such a pipe installation can be the main coolant loop of a pressurized-water reactor installation, where each leg of the loop must be formed by a pipe having one end rigidly connected to the reactor pressure vessel and the other end rigidly connected with the primary header of a steam generator, the cold leg having its usual interposed main coolant pump. The pipe carries the pressurized-water coolant and is, therefore, subject to axial movement due to the thermal changes inherently involved by the operation of the reactor. To permit this movement, it is desirable that such main coolant loop pipes be as free from supports between their ends as is consistent with their support against undue sagging due to their weight, which includes that of the water coolant they carry, and considering their relatively great length as compared to their diameter.
It follows that in many cases the pipe between its supporting positions forms a free span representing a vibratory system having an inherent resonance involving low frequencies and consequently large amplitudes. If the system is disturbed as by receiving a vibratory force, the pipe can be made to oscillate between its supported positions at a low frequency and consequently a great amplitude excessively stressing the pipe's wall which is, of course, a solid wall and not designed to be flexed to any great degree.
The results of such oscillations of large amplitude can be very serious. In the case of the reactor installation referred to, it must be designed in anticipation of the remote chance that pipes might receive the force of seismic disturbances, such as an earthquake, in which event one or more of the main coolant loop pipes might be excited to such a degree that it oscillates with such a large amplitude as to stress the pipe wall to a degree ultimately resulting in a rupture, releasing the pressurized-water coolant which might possibly be radioactive.
The prior art has proposed the attachment of a mass to such a vibratory pipe span, the mass being connected between the supported ends of the span, to detune the vibratory system involved. However, in many cases, where the pipe length is great as compared to its diameter, particularly with the pipe containing a liquid, such as water, the size of the mass required to provide an effective result, is too large to be considered as a practical solution.